This invention relates generally to the treatment of injured, degenerated, or diseased tissue in the human spine, especially failed intervertebral discs and diseased vertebrae. It further relates to the removal of damaged tissue and to the stabilization of the remaining spine by bony fusion of at least two vertebrae adjacent or nearly adjacent to the space left by the surgical removal of tissue. More particularly, this invention relates to the implantation of devices which can structurally replace the removed discs during healing and at the same time share compressive load to facilitate growth of new bone between adjacent vertebrae. This invention further relates to the implantation of devices which are compatible with the natural curvature of the spinal column.
For many years a treatment, often a treatment of last resort, for serious back problems has been spinal fusion surgery. Disc surgery, for example, typically requires removal of a portion or the entirety of an intervertebral disc. In such cases the structural contribution of the removed disc must be replaced. The most common sites for such surgery, namely those locations where body weight most concentrates its load, are the lumbar discs in the L1-2, L2-3, L3-4, L4-5, and L5-S1 intervertebral spaces. In addition, a number of degenerative diseases and other conditions such as scoliosis require correction of the relative orientation of vertebrae by surgery and fusion.
In current practice, a surgeon will use one or more procedures known in the art to attempt to fuse remaining adjacent spinal vertebrae. Some of the prior art techniques used with at best mixed success have been described in Nicholson, et al., U.S. Pat. No. 6,096,080, issued Aug. 1, 2000, which is incorporated by reference as though fully set forth herein. That patent set forth a successful advance over such prior art.
Prior art methods of attempting to achieve fusion as described in Nicholson, et al. do not produce reliable and predictable results. For example, Fraser, R. D. points out in xe2x80x9cInterbody, Posterior and Combined Fusions,xe2x80x9d Spine, V20(24S):1675, Dec. 15, 1995: xe2x80x9c[A]nalysis of the literature does not indicate that one form of fusion is significantly better than another for degenerative conditions of the lumbar spine.xe2x80x9d Ray, Charles D., reported the results of the original IDE study involving his Ray Threaded Fusion Cage (Ray-TFC) in Spine V22(6):667, Mar. 15, 1997. The study reported that of the two hundred eight patients who had two year follow-up, 96% had fusion but with only 40% having excellent results and 25% having fair or poor results.
There is a strong current of opinion among leading spine surgeons that loading the material which is to become bone and produce fusion with a compressive mechanical load yields superior results both in strength and in brevity of healing time. Such loading works in conjunction with the standard procedure of preparing the vertebrae to be fused by breaking through, or cutting into, the hardened endplate surfaces of vertebral bone so as to allow an interposed bone graft or implant to come into direct contact with vascularized cancellous bone tissue. The latter enables blood flow through material placed in the intervertebral space which in turn initiates the growth of new bone across the intervertebral space. This process allows for the incorporation of inserted bone grafts or implants into the two respective adjacent vertebral surfaces such that they become one continuous and rigid segment of bone.
In addition, the restoration of normal anatomy is a basic principle of all orthopedic reconstructive surgery. Lordosis, a pronounced forward curvature of the lumbar region of the spine, is a factor relevant to the design of lumbar implants. It is known in the art that the preservation of the natural curvature of the lumbar spine requires a design having a modest taper approximately equivalent to the effective angularity of the removed tissue.
Therefore there is a perceived need for a device which simultaneously provides: mechanical stability upon implantation; a proper substrate such as allograft bone to induce the growth of bone across the implant region; compressive loading to the implant so as to enhance bone growth and calcification; and accommodation to the normal anatomical curvature of the spine. There is also a perceived need for an implant configuration that can be used in other regions of the spine in addition to the lumbar region, specifically in the cervical region.
Bone based implants may have cylindrical, rectangular, and generic shapes. In the past, Cloward, Wilterberger, Crock, Viche, Bagby, Brantigan, and others have taught various methods involving the drilling of holes across the disc space between two adjacent vertebrae of the spine for the purpose of causing an interbody spinal fusion upon placement of a dowel in the hole. Cloward, for instance, taught placing a dowel of bone within a drilled hole for the purpose of bridging the intervertebral gap and incorporating the dowel into the fusion. Viche taught the threading of that bone dowel.
Bone used in dowel implants can be obtained from the patient""s own hip iliac crest or it can be obtained as allograft from a tissue processor such as Regeneration Technologies, Inc. Many prior art bone grafts required distraction of the intervertebral space and contouring of the bone graft to ride on the end plates of the vertebral bodies.
Dowel bone grafts have been described in, for example, the following U.S. Patents: Michelson in U.S. Pat. Nos. 5,015,247, 5,860,973, and 6,149,650; Grivas, et al., in U.S. Pat. No. 5,814,084; Zdeblick, et al., in U.S. Pat. Nos. 6,206,922 and 6,245,072; Koros in U.S. Pat. No. 6,217,579; and McKay in U.S. Pat. No. 6,261,586.
Michelson, in U.S. Pat. No. 5,015,247 (May 14, 1991), describes a xe2x80x9cthreaded spinal implantxe2x80x9d which would plausibly be a dowel type design with threads. Grivas, et al., in U.S. Pat. No. 5,814,084 (Sep. 29, 1998), describe a dowel made from xe2x80x9ca plug from the shaft (diaphysis) of various long bones.xe2x80x9d Grivas et al. state that their dowel has improved biomechanical and vertebral fusion induction properties compared to standard dowels known in the art.
Michelson, in a later patent, U.S. Pat. No. 5,860,973 (Jan. 19, 1999), teaches a spinal implant design which is xe2x80x9cat least in part cylindrical in shape.xe2x80x9d However, the use of a single dowel bone graft implant tends to flatten the intervertebral space, causing it to be distracted in such a way as to induce an undesirable change in spinal curvature. Also, the instrumentation used to implant single dowels is bulky and tends to be difficult to use partly because it blocks direct view of the implantation site. The Cloward method is now mostly of historical interest and has been largely replaced by methods such as the Smith-Robinson technique in which a straightforward discectomy is followed by implantation of a square block of bone. Smith-Robinson has been until recently a standard, even though it has problems related to both (1) bone non-incorporation, mostly due to poorly interfaced surfaces since the end plate of the vertebral bodies is not a square configuration, and (2) collapse or telescoping of the graft within the patient""s adjacent vertebral body causing pain. Such complications may be attributed to poor fit of the graft and inadequate preparation of the endplates to accept the graft.
The invention of Nicholson, et al., U.S. Pat. No. 6,096,080 made great strides in overcoming the many disadvantages of prior art devices and procedures. However, it nevertheless had some drawbacks which are addressed by the current invention. The dovetail implant of that invention can only be placed in one orientation if it is to achieve its full intended effect. Moreover, it requires a relatively wide distraction of the adjacent vertebrae to accommodate the height of the implant necessitated by the geometry of the dovetail configuration.
It is therefore an object of the current invention to provide a spinal fusion implant able to provide a substrate suitable to the induction of new bone growth between the vertebral bodies being fused. It is a further object to provide a device which promotes bone growth between vertebrae adjacent to the space left by the excised material by progressive sharing of the compressive load to the bone graft inserted within the device. It is a further object to create an implantable device for stabilizing the spine by limiting relative motion between the involved vertebrae in torsion loading during healing. It is yet a further object to provide mechanical stability between adjacent vertebrae while bone grows across the intervertebral space and while simultaneously maintaining the natural lordosis of the cervical or lumbar spine. It is a further object of the invention to provide a device which avoids or minimizes interference with various imaging technologies. It is yet another object of this invention to be capable of being fabricated from human bone allograft material. It is yet a further object of this invention to provide an implant which can be inserted with varying orientation. It is yet another object of this invention to provide an implant which provides interlocking capability but with a variable separation between the interlocking portions to allow for lesser distraction of adjacent vertebrae where such diminished distraction is necessary. It is yet another object of the invention to provide an implant which is also useful in the repair of cervical intervertebral defects.
The present invention is a design, in six embodiments, for a double dowel configuration of an intervertebral graft implant made of bone, preferably allograft bone, for use in the fusion of adjacent vertebral bodies. It further comprises three embodiments of a vertebral distractor and reamer guide intended for use in the preparation of the implantation site.
In its simplest form, the invention is a bone graft implant having a unitized shape of two conjoined circular cylindrical dowels whose major axes are laterally spaced apart by a distance that is less than half the sum of the diameters of the two circular cylindrical dowels at all points along the major length of the implant. The major axes of the conjoined circular cylindrical dowels are coplanar, and in this simplest embodiment the diameters of the two conjoined circular cylindrical dowels are equal. For cervical implantation, the diameter of each conjoined circular cylindrical dowel is between about 7 millimeters and about 14 millimeters. For lumbar implantation, the diameter of each conjoined circular cylindrical dowel is between about 10 millimeters and about 22 millimeters. When implanted, the plane in which the dowels are coplanar is perpendicular to the sagittal plane of the patient""s spine.
A second embodiment of the double dowel design is characterized by a unitized shape of two conjoined circular cylindrical dowels whose major axes are laterally spaced apart by a distance that is less than half the sum of the diameters of the two circular cylindrical dowels at all points along the major length of the implant. The major axes of the conjoined circular cylindrical dowels are coplanar, and each double dowel set has a lordotic taper cut at one end. The diameters of the two conjoined circular cylindrical dowels are equal. For cervical implantation, the diameter of each conjoined circular cylindrical dowel is between about 7 millimeters and about 14 millimeters. For lumbar implantation, the diameter of each conjoined circular cylindrical dowel is between about 10 millimeters and about 22 millimeters. When implanted, the plane in which the dowels are coplanar is perpendicular to the sagittal plane of the patient""s spine.
A third embodiment of the double dowel design is characterized by a unitized shape of two conjoined truncated conical dowels whose major conical axes are laterally spaced apart at all points along the length of the major dimension of the implant by a distance that is less than half the sum of the diameters of the two truncated conical dowels at all points along the length of the major dimension of the implant. The major axes of the conjoined truncated conical dowels are coplanar. The diameters of the two conjoined truncated conical dowels are equal at all points along the length of the major dimension of the implant. For cervical implantation, the major base diameter of each conjoined truncated conical dowel is between about 7 millimeters to about 14 millimeters. For lumbar implantation, the major base diameter of each conjoined truncated conical dowel is between about 10 millimeters to about 22 millimeters. When implanted, the plane in which the dowels are coplanar is perpendicular to the sagittal plane of the patient""s spine.
In a fourth embodiment, the double dowels are circular cylinders having unequal diameters. When implanted, the plane in which the dowels are coplanar is closer to being perpendicular to the sagittal plane of the patient""s spine than parallel to the sagittal plane.
In a fifth embodiment of the double dowel design is characterized by a unitized shape of two conjoined circular cylindrical dowels whose major axes are laterally spaced apart by a distance that is less than half the sum of the diameters of the two circular cylindrical dowels at all points along the major length of the implant. The major axes of the conjoined circular cylindrical dowels are coplanar, and the diameters of the two conjoined circular cylindrical dowels are equal. For cervical implantation, the diameter of each conjoined circular cylindrical dowel is between about 7 millimeters and about 14 millimeters. For lumbar implantation, the diameter of each conjoined circular cylindrical dowel is between about 10 millimeters and about 22 millimeters. When implanted, the plane in which the dowels are coplanar is parallel to the sagittal plane of the patient""s spine.
In a sixth embodiment of the double dowel design is characterized by a unitized shape of two circular cylindrical dowels conjoined by an intervening planar bridge. The major axes of the conjoined circular cylindrical dowels are coplanar, and the diameters of the two conjoined circular cylindrical dowels are equal. For cervical implantation, the diameter of each conjoined circular cylindrical dowel is between about 7 millimeters and about 14 millimeters. For lumbar implantation, the diameter of each conjoined circular cylindrical dowel is between about 10 millimeters and about 22 millimeters. When implanted, the plane in which the dowels are coplanar is parallel to the sagittal plane of the patient""s spine.
A first embodiment of the vertebral distractor for separating and maintaining separation of two adjacent vertebral bodies has two tapers disposed in lateral separation from one another and having a cavity comprised of two parallel and overlapping circular cylindrical bores penetrate the main body therethrough. The distractor has a curved land having a radius of curvature approximately equal to the radius of curvature of the anterior faces of vertebral bodies.
A second embodiment of the vertebral distractor used for separating and maintaining separation of two adjacent vertebral bodies has two portions, one of which is an outer body having a rectangular hole and two tapers disposed in lateral separation from one another, and a curved land having a radius of curvature approximately equal to the radius of curvature of the anterior faces of vertebral bodies. The other portion is an insertable reamer guide consisting of a rectangular solid having within itself two parallel and overlapping circular cylindrical bores comprising a single hole such that the overlapping circular cylindrical holes have axes that are separated by less than the diameter of either circular cylindrical hole with the axes of the two holes being disposed symmetrically within the two faces of the rectangular solid to which the axes of the holes are perpendicular.
A third embodiment of the vertebral distractor used for separating and maintaining separation of two adjacent vertebral bodies has two portions, one of which an outer body having a rectangular hole. It has two tapers disposed in lateral separation from one another and a curved land having a radius of curvature approximately equal to the radius of curvature of the anterior faces of vertebral bodies. The other portion is an insertable reamer guide consisting of a rectangular solid having within itself a cylindrical hole with an axis that is perpendicular to two rectangular faces of the rectangular solid and parallel to the four other rectangular faces of the rectangular solid and located such that the axis of the hole is separate from an axis is defined by a line passing between respective centroids of the two rectangular faces to which the axis is perpendicular with the separation between the axis of the hole and the axis being less than the radius of the hole.